The ability to structure and pattern silicon is important for many applications. There has been particular interest in patterning silicon to make nanostructures. Relevant information regarding silicon fabrication processes known to those of skill in the art can be found, for example, in Sami Franssila, Introduction to Microfabrication (John Wiley & Sons, 2004), and the references cited there.
Semiconductor nanowires have become the focal point of research over the last decade due to their interesting physical, chemical and biological properties. There is particular interest surrounding silicon nanowires, as silicon is one of the most abundant materials in the earth's crust and has become a cornerstone for many of the electronic, optoelectronic, electro-chemical and electro-mechanical devices upon which designs are based.
Today, many nanosystems are not utilized commercially due to the large cost associated with fabrication, and limitations in the scalability of nanowire synthesis. Nanowires have been grown bottom up using molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), and physical vapor deposition (PVD). They have also been fabricated top-down using techniques like reactive ion etching (RIE) and inductively coupled plasma (ICP). These systems require high temperature and/or low pressure which is largely responsible for the high cost. A push towards solution based techniques that can be operated in ambient conditions is important given their low cost, simplicity of design and ease of utilization.
Recent work has demonstrated the fabrication of silicon nanowires using a solution made up of a metal salt and a strong acid (typically AgNO3 and HF). (See reference (a).) By controlling the concentrations of each component in solution, silicon can be etched normal to the plane of the wafer forming vertically aligned silicon nanowires with an average diameter of 150 nm and a diameter range from 20-300 nm. Through the realization that silver is precipitating out of solution and catalyzing the silicon etch, the technique has been modified to incorporate the addition of H2O2 into the chemical bath and Ag metal directly deposited onto silicon. Polystyrene spheres of uniform dimensions were dispersed prior to the deposition of the Ag in order to use them as an etch mask and define the nanowire. (See reference (e).) As a result, ordered arrays of silicon nanowires with a homogeneous diameter and length were demonstrated.
The ultimate diameter achieved with this technique has been limited. The ability to achieve sub-100 nm dimensions is of value to a variety of electronic, optoelectronic, electrochemical and electromechanical applications. For example, it is within the sub-100 nm range that silicon begins to demonstrate novel properties distinguishable from the properties of bulk silicon. In addition, an increase in surface area at the low nanometer scale is of value.